16-QAM
16-QAM (Quadrature Amplitude Modulation) is a digital modulation scheme which is commonly used for example in IMT 2000 based mobile communication systems, such as UMTS or CDMA 2000. The 16 modulation symbols are defined by distinct points in the complex signal space in which the 16-QAM constellation is commonly illustrated. Each of these points represents one 16-QAM symbol.
For binary information transmission systems, four different bits may be used to determine one of the existing 16-QAM symbols. Therefore one 16-QAM symbol consists (or can be represented by a word) of 4 bits, and is represented by a complex value in the complex plane. Generally the complex value of a modulation symbol can be represented by its cartesian inphase- and quadrature-components (I and Q components) relative to the respective I-axis and Q-axis in the complex plane. These axes also divide the complex plane in four quadrants. The representation of a modulation symbol by its real and imaginary part in the complex plane is equivalent to its representation by polar components, i.e. radius and angle.
For a better understanding of the invention, it is assumed here a specific constellation of the 16-QAM symbols, where the signal points within a quadrant of the complex plane are arranged such that they form a square of four points in two orthogonal directions of the signal space. Consequently such a mapping is commonly known as square 16-QAM or lattice 16-QAM. Two examples are given in FIG. 1 and FIG. 2.
The invention assumes that the 16-QAM symbols are arranged using a square 16-QAM mapping. It should be apparent to the skilled person that for each rotated 16-QAM constellation as for example shown in FIG. 2, the axes of the complex plane may be chosen such that the rotated 16-QAM constellation can be viewed as in FIG. 1.
Commonly, the so-called Gray mapping is used to associate the 16 modulation symbols in a 16-QAM constellation with a quadruple of bits which is mapped to the respective symbol. According to this Gray mapping scheme, adjacent modulation symbols in the horizontal or vertical direction differ in one bit only.
16-QAM Subset Partitioning
Generally the set of symbols within a constellation may be partitioned into subsets to define the symbol regions that correspond to the logical value of a certain bit. Since for a 16-QAM constellation 4 bits are relevant, there are four subsets, one for each bit. Each subset may be further divided into two symbol regions that correspond to the two logical values of the respective bit in the corresponding subset.
Obviously, there exist various subset partitions. However some of these are equivalent for example from the viewpoint of error rate performance. Still there exist certain partitioning schemes that are more widely used than others. Four examples of subset partitioning schemes are given for example in Chindapol, A.; Ritcey, J. A., “Design, analysis, and performance evaluation for BICM-ID with square QAM constellations in Rayleigh fading channels”, IEEE Journal on Selected Areas in Communications, Volume: 19, Issue: 5, May 2001, Pages: 944-957 and also in FIG. 11 for so -called Gray mapping.
Constellation Rearrangement for 16-QAM Gray Mapping
For Gray mapping, it has been shown that a constellation rearrangement approach improves the performance if two or more versions of the same word are transmitted. The constellation rearrangement scheme for Gray mapping is based on different levels of reliability for the bits, depending on the position of the selected 16-QAM symbols within the constellation. Consequently the rearrangement rules focus on changing the location of the rearranged version of the 16-QAM symbol to achieve an averaging effect of the levels of reliability. For details on constellation rearrangement for 16-QAM Gray mapping, it is referred to the granted patent EP 1,293,059 B1 or the publication WO 2004/036817 A1 of the applicant.
Transmit Diversity Schemes
There exist several well known transmit diversity techniques. The term “transmit diversity” as used in this document describes the transmission of one or several versions relating to identical data on several (at least two) diversity branches. For example the following schemes are considered as transmit diversity (see e.g. J. D. Gibson, “The Mobile Communications Handbook”, IEEE Press, 1996, Chapter 12.2):                Site Diversity: The transmitted signal originates from different sites, e.g. different base stations in a cellular environment.        Antenna Diversity: The transmitted signal originates from different antennas, e.g. different antennas of a multi antenna base station.        Polarization Diversity: The transmitted signal is mapped onto different polarizations.        Frequency Diversity: The transmitted signal is mapped e.g. on different carrier frequencies or on different frequency hopping sequences.        Time Diversity The transmitted signal is e.g. mapped on different interleaving sequences. This includes ARQ schemes that re-transmit data upon request.        Code Diversity The transmitted signal is mapped on different codes in e.g. a CDMA (Code Division Multiple Access) system.        
In the above referenced application and patent of the applicant respectively, it has been shown that the use of constellation rearrangement schemes together with transmit diversity may significantly improve the bit-error rate of a transmitted signal in mobile communication environments. It is shown to be optimum considering four different constellations for 16-QAM Gray mapping. Nevertheless, there is still a demand for an optimization of modulation and coding schemes used for communications, in particular in a mobile communication environment, to reduce the number of required constellations or to improve the achieved error performance.